A WeakInvalidationListener can be used, if an Observable
should only maintain a weak reference to the listener. This helps to avoid
memory leaks, that can occur if observers are not unregistered from observed
objects after use.

WeakInvalidationListener are created by passing in the original
InvalidationListener. The WeakInvalidationListener should
then be registered to listen for changes of the observed object.

Note: You have to keep a reference to the InvalidationListener, that
was passed in as long as it is in use, otherwise it will be garbage collected
to soon.